Screening of mitochondrial function in the presence of drug compounds is becoming of increased interest because, as more research accumulates for fundamental drug mechanisms and fundamental causes of disease, more is being discovered that points directly to mitochondria function. [Boelsterli et al., Drug Discovery Today 13, 982-988, (2008); Degli Esposti et al., Biochem. J. 301, 161-167 (1994); Dykens et al., Drug Discovery Today 12, 777-785, (2007); Kovacic et al., Current Medicinal Chemistry 12, 2601-2623 (2005); Mabalirajan et al., J. Immunol. 181, 3540-3548 (2008); Mabalirajan et al., J. Immunol. 183, 2059-2067 (2009); Nadanaciva et al., Current Protocols in Toxicology 2, 1-9, (2009); Petit et al., Trends in Pharmacological Sciences 26, 258-264 (2005); Scatena et al., Am. J. Physiol. 293, C12-C21, (2007); Wang et al., J. Neuroscience 28, 9473-9485 (2008); Yang et al., Int. Immunopharmacol. 7, 1411-1421 (2007).]
One classic example is the treatment of bipolar disorder with lithium salts. Until recently, the fundamental mechanism of action has been unknown. The use of lithium salts can be traced back over 100 years ago for the treatment of mania. In 1970, the FDA approved its use as a treatment for bipolar disorder, and it has been heavily used ever since. Clinically, the mechanism was traced to the cell/tissue level, but the molecular biology was still uncertain.
In the last few years, research has shown that the lithium ion stimulates oxidative phosphorylation in the mitochondria by acting as an activator for the electron transport chain [Maurer et al., Bipolar Disorders 11, 515-522 (2009)]. In another example, several potent analgesics, including Demerol and barbiturates, reversibly inhibit Complex I of the electron transport chain causing the nerve cells to not be as active thereby reducing the pain an individual is experiencing [Hatefi et al., J. Biol. Chem. 244, 2358-2365 (1969)]. In cancer cells, the mitochondrial activity is suppressed because most of the metabolism is routed through glycolysis to produce lactate. This suppression also causes the programmed cell death, which is caused by the cytochrome release of the mitochondria to be directly suppressed thereby allowing the cancer cell to achieve its out of control state. If a therapeutic compound is targeted to specific complexes and proteins of the mitochondria then the therapeutic could effectively gain control of the mitochondria and stabilize the cell function or allow the cell to achieve programmed cell death, as in the case of cancer.
It is clear mitochondria are important to cell function due to their important role in energy conversion, but they are also important in calcium homeostasis, heme synthesis, steroid synthesis, and programmed cell death [Wojtczak et al., Basic Mitochondrial Physiology in Cell Viability and Death, John Wiley & Sons, New York, 1-36 (2008)]. Mitochondrial dysfunction is increasingly being found to be the cause of drug-induced toxicities. Pharmaceutical companies are therefore realizing the importance of early identification of drug effects of mitochondrial function in order to avoid late-stage attrition during drug development [Dykens et al., Drug Discovery Today 12, 777-785 (2007); Dykens et al., Expert Review of Molecular Diagnostics 7, 161-175 (2007)]. Many screening strategies are being developed to screen chemical drug libraries early in the drug development cycle for mitochondrial dysfunction.
Mitochondrial screening has not been part of the preclinical drug development process for four primary reasons: (1) it is difficult to accurately assess mitochondrial function; (2) there is no high throughput method for screening drug candidates; (3) drug companies did not fully understand how common mitochondrial dysfunction was a cause of drug toxicity; and the lack of evidence as to whether in vitro responses correspond to in vivo outcomes [Dykens et al., Drug Discovery Today 12, 777-785 (2007); Dykens et al., Expert Review of Molecular Diagnostics 7, 161-175 (2007)]. Although there are a variety of methods that are used to study mitochondrial function/dysfunction from animal models, these methods are time consuming and do not lend themselves to high throughput methods because it takes time for cell or organism growth, drug treatment, and waiting for cell death. Moreover, cell death is not necessarily the best method for studying mitochondrial dysfunction because it is an indirect method.